For an efficient production of a subsurface hydrocarbon source, reservoir engineers and exploration geoscientists aim to locate the best spots and trajectories for the wells to be drilled in and/or for the fracking of a shale formation.
Shale gas or oil formations, typically are heterogeneous formations whose mineralogy, organic content, natural fractures, and other properties vary from place to place with variations in the geological formations and rock properties.
A shale oil and/or shale gas play, in contrast to conventional oil and gas reservoirs or sources, which are generally more compact and limited in their areal surface and for which classical exploration and production techniques have proven to be effective, spreads over a large area.
A well drilled in a conventional oil and gas reservoir is capable of draining oil or gas over a relatively large area of the reservoir. In contrast, shale oil and shale gas plays require a number of relatively closely spaced wells, all of which are situated over the area of the shale oil and/or shale gas play, to produce the play successfully. Some of these wells produce significant larger volumes, i.e. highly productive wells or zones, also called ‘sweet spots’, compared to others, i.e. less productive wells or zones. Identification of the position of sweet spots before drilling and/or fracking will save considerable time and money, as drilling is preferably limited or essentially limited to the geographical location or zone indicative of and associated with the presence of a sweet spot at a shale oil and/or shale gas play.
Subsurface reservoirs are typically modelled from seismic and geological data obtained by geologists, geophysicists, and petrophysicists, using dedicated programmed computers and geostatistic modelling, for example. To produce shale sources identified by geostatistic modelling, in practice, a technique known as grid drilling is applied. That is, the area to be explored is overlaid by a virtual grid identifying geographic locations of the area. At a plurality or all of the grid positions wells are drilled to the shale formation and hydrocarbon productivity of such well is measured and expressed in a quantity indicative of the respective hydrocarbon productivity.
As well drilling is both time and cost consuming, in practice, production engineers aim to minimize drilling and/or fracking activities to the extend necessary for an efficient winning of oil or gas from an underground formation.
As an alternative for, or in addition to the seismic and geological data, in particular for detecting subsurface hydrocarbon sources, microbial prospecting may be used. It is known that certain bacteria respond to hydrocarbons, and hence may be used as indicators for finding oil and gas reservoirs.
Microbial prospecting for hydrocarbons is a surface exploration technique, based on the premise that volatile, gaseous hydrocarbon components such as methane (C1), ethane (C2), propane (C3) and butane (C4), for example, migrate upward from subsurface hydrocarbon accumulations through natural microcracks in geologic structures, generally called microseepage, and may affect microorganisms present in the shallow sub-soil ecosystem. Hydrocarbon-oxidizing bacteria exclusively use these gases as a carbon source for their metabolic activities and growth. These bacteria are typically found enriched in the soils/sediments above hydrocarbon bearing structures and may differentiate between hydrocarbon prospective and non-prospective productivity areas. That is, in terms of a shale gas or shale oil reservoir, between sweets spots and non- or low-productive zones, respectively.
The detection of various groups of methane, ethane, propane or butane oxidizing bacteria, for example, in the surface soils or sediments, helps to evaluate the prospects for hydrocarbon exploration and production.
Microbial prospecting essentially involves the collection of sub-soil samples from an area under investigation, followed by isolation and enumeration of microbial activity found in the samples. The microorganisms involved are not restricted to hydrocarbon-oxidizing or metabolising microorganisms, but may also include microorganisms that tolerate the volatile components, i.e. non-metabolising microorganisms, or microorganisms that are, to a certain extend, degraded or even eliminated by microseepage of volatile hydrocarbons.
Microorganisms present in samples of the surface soil or sediment taken from a play or area under investigation may be detected by microbiological screening techniques known in standard molecular biology techniques, providing microbial data identifying microorganisms, such as DNA sequences that serve as so-called biomarkers.
Analysing the microbial activity found, in terms of population density or bacterial count, for example, and applying a statistical approach and standard deviation value taken as a background value for the demarcation of anomalous zones, the results of the microbial analysis are presented on a geographical map of the area under investigation. This map serves the function of a predictive production map for drilling wells in the earth formations for the production of oil and/or gas.
For presalt hydrocarbon reservoirs, for example, at present, microbial prospecting appears to be a feasible exploration technique for prospecting the location of wells to be drilled to exploit an oil or gas source from such presalt reservoirs.
The results obtained by microbial prospecting may be integrated with other geoscientific and geophysical data to predict the hydrocarbon prospectivity of an area under investigation.
Although work on microbial prospecting has started in the early years of the twentieth century, and since then plural bacterial populations have been identified the presence and/or absence of which qualify for indicating the presence and/or absence of subsurface hydrocarbon sources, the effectiveness of microbial prospecting of heterogeneous subsurface hydrocarbon sources, such as shale gas and shale oil sources, still necessitates improvement. It is in particular the heterogeneity of a shale oil or gas play that makes exploration critical for maximizing production efficiency from such plays by minimizing the number of wells to be drilled.